10:49 a.m.
On 3/25/22 12:29 PM, Daniel P. Berrangé wrote:
On Fri, Mar 18, 2022 at 02:34:29PM +0100, Claudio Fontana wrote:
> On 3/17/22 4:03 PM, Dr. David Alan Gilbert wrote:
>> * Claudio Fontana (cfontana(a)suse.de) wrote:
>>> On 3/17/22 2:41 PM, Claudio Fontana wrote:
>>>> On 3/17/22 11:25 AM, Daniel P. Berrangé wrote:
>>>>> On Thu, Mar 17, 2022 at 11:12:11AM +0100, Claudio Fontana wrote:
>>>>>> On 3/16/22 1:17 PM, Claudio Fontana wrote:
>>>>>>> On 3/14/22 6:48 PM, Daniel P. Berrangé wrote:
>>>>>>>> On Mon, Mar 14, 2022 at 06:38:31PM +0100, Claudio Fontana
wrote:
>>>>>>>>> On 3/14/22 6:17 PM, Daniel P. Berrangé wrote:
>>>>>>>>>> On Sat, Mar 12, 2022 at 05:30:01PM +0100, Claudio
Fontana wrote:
>>>>>>>>>>> the first user is the qemu driver,
>>>>>>>>>>>
>>>>>>>>>>> virsh save/resume would slow to a crawl with
a default pipe size (64k).
>>>>>>>>>>>
>>>>>>>>>>> This improves the situation by 400%.
>>>>>>>>>>>
>>>>>>>>>>> Going through io_helper still seems to incur
in some penalty (~15%-ish)
>>>>>>>>>>> compared with direct qemu migration to a nc
socket to a file.
>>>>>>>>>>>
>>>>>>>>>>> Signed-off-by: Claudio Fontana
<cfontana(a)suse.de>
>>>>>>>>>>> ---
>>>>>>>>>>> src/qemu/qemu_driver.c | 6 +++---
>>>>>>>>>>> src/qemu/qemu_saveimage.c | 11 ++++++-----
>>>>>>>>>>> src/util/virfile.c | 12 ++++++++++++
>>>>>>>>>>> src/util/virfile.h | 1 +
>>>>>>>>>>> 4 files changed, 22 insertions(+), 8
deletions(-)
>>>>>>>>>>>
>>>>>>>>>>> Hello, I initially thought this to be a qemu
performance issue,
>>>>>>>>>>> so you can find the discussion about this in
qemu-devel:
>>>>>>>>>>>
>>>>>>>>>>> "Re: bad virsh save /dev/null
performance (600 MiB/s max)"
>>>>>>>>>>>
>>>>>>>>>>>
https://lists.gnu.org/archive/html/qemu-devel/2022-03/msg03142.html
>>>>>
>>>>>
>>>>>> Current results show these experimental averages maximum
throughput
>>>>>> migrating to /dev/null per each FdWrapper Pipe Size (as per QEMU
QMP
>>>>>> "query-migrate", tests repeated 5 times for each).
>>>>>> VM Size is 60G, most of the memory effectively touched before
migration,
>>>>>> through user application allocating and touching all memory with
>>>>>> pseudorandom data.
>>>>>>
>>>>>> 64K: 5200 Mbps (current situation)
>>>>>> 128K: 5800 Mbps
>>>>>> 256K: 20900 Mbps
>>>>>> 512K: 21600 Mbps
>>>>>> 1M: 22800 Mbps
>>>>>> 2M: 22800 Mbps
>>>>>> 4M: 22400 Mbps
>>>>>> 8M: 22500 Mbps
>>>>>> 16M: 22800 Mbps
>>>>>> 32M: 22900 Mbps
>>>>>> 64M: 22900 Mbps
>>>>>> 128M: 22800 Mbps
>>>>>>
>>>>>> This above is the throughput out of patched libvirt with multiple
Pipe Sizes for the FDWrapper.
>>>>>
>>>>> Ok, its bouncing around with noise after 1 MB. So I'd suggest
that
>>>>> libvirt attempt to raise the pipe limit to 1 MB by default, but
>>>>> not try to go higher.
>>>>>
>>>>>> As for the theoretical limit for the libvirt architecture,
>>>>>> I ran a qemu migration directly issuing the appropriate QMP
>>>>>> commands, setting the same migration parameters as per libvirt,
>>>>>> and then migrating to a socket netcatted to /dev/null via
>>>>>> {"execute": "migrate", "arguments":
{ "uri", "unix:///tmp/netcat.sock" } } :
>>>>>>
>>>>>> QMP: 37000 Mbps
>>>>>
>>>>>> So although the Pipe size improves things (in particular the
>>>>>> large jump is for the 256K size, although 1M seems a very good
value),
>>>>>> there is still a second bottleneck in there somewhere that
>>>>>> accounts for a loss of ~14200 Mbps in throughput.
>>>
>>>
>>> Interesting addition: I tested quickly on a system with faster cpus and
larger VM sizes, up to 200GB,
>>> and the difference in throughput libvirt vs qemu is basically the same ~14500
Mbps.
>>>
>>> ~50000 mbps qemu to netcat socket to /dev/null
>>> ~35500 mbps virsh save to /dev/null
>>>
>>> Seems it is not proportional to cpu speed by the looks of it (not a totally
fair comparison because the VM sizes are different).
>>
>> It might be closer to RAM or cache bandwidth limited though; for an extra copy.
>
> I was thinking about sendfile(2) in iohelper, but that probably
> can't work as the input fd is a socket, I am getting EINVAL.
Yep, sendfile() requires the input to be a mmapable FD,
and the output to be a socket.
Try splice() instead which merely requires 1 end to be a
pipe, and the other end can be any FD afaik.
With regards,
Daniel
I did try splice(), but performance is worse by around 500%.
It also fails with EINVAL when trying to use it in combination with O_DIRECT.
Tried larger and smaller buffers, flags like SPLICE_F_MORE an SPLICE_F_MOVE in any
combination; no change, just awful performance.
Here is the code:
#ifdef __linux__
+static ssize_t safesplice(int fdin, int fdout, size_t todo)
+{
+ unsigned int flags = SPLICE_F_MOVE | SPLICE_F_MORE;
+ ssize_t ncopied = 0;
+
+ while (todo > 0) {
+ ssize_t r = splice(fdin, NULL, fdout, NULL, todo, flags);
+ if (r < 0 && errno == EINTR)
+ continue;
+ if (r < 0)
+ return r;
+ if (r == 0)
+ return ncopied;
+ todo -= r;
+ ncopied += r;
+ }
+ return ncopied;
+}
+
+static ssize_t runIOCopy(const struct runIOParams p)
+{
+ size_t len = 1024 * 1024;
+ ssize_t total = 0;
+
+ while (1) {
+ ssize_t got = safesplice(p.fdin, p.fdout, len);
+ if (got < 0)
+ return -1;
+ if (got == 0)
+ break;
+
+ total += got;
+
+ /* handle last write truncate in direct case */
+ if (got < len && p.isDirect && p.isWrite &&
!p.isBlockDev) {
+ if (ftruncate(p.fdout, total) < 0) {
+ return -4;
+ }
+ break;
+ }
+ }
+ return total;
+}
+
+#endif
Any ideas welcome,
Claudio